Systems and methods for guiding an instrument using haptic object with collapsing geometry

ABSTRACT

A computer-implemented method for guiding an instrument, comprises determining, by a processor associated with a computer, a current orientation angle of an instrument axis relative to a target axis. The method also comprises establishing, by the processor, a haptic boundary associated with the instrument based on the determined orientation angle of the instrument axis relative to the target axis. The haptic boundary is configured to constrain the instrument axis from being moved to an angle substantially greater than the current orientation angle.

TECHNICAL FIELD

The present disclosure relates generally to haptic guidance systems and,more particularly, to systems and methods for guiding a surgicalinstrument using a haptic object having a collapsible geometry.

BACKGROUND

Many surgical procedures require the use of specialized tools to performsurgical tasks with a high degree of accuracy and precision. In somecases, such surgical procedures require precise positioning and/orplacement of the tool at or near a particular point within a patient'sanatomy. For example, many orthopedic procedures rely on the accurateplacement of pins, screws, guide and/or post holes, or other elements ina precise position and orientation with respect to an anatomical featureof the patient. In order to ensure that these elements are properlypositioned and oriented, great care is required on the part of thesurgeon to ensure that the surgical tool(s) (e.g., drill, saw, reamer,etc.) used to position these elements is precisely and accuratelyaligned with the anatomy of the patient. However, this can beparticularly challenging without the use of a guide and even morechallenging in minimally-invasive procedures where visibility at thesurgical site is limited or, in some cases, nonexistent.

Early solutions for enhancing the accuracy and precision of thealignment of tools in a surgical environment involved the use ofmechanical guide elements, such as jigs. These mechanical guides weretypically placed and/or mounted in close proximity to the anatomy of thepatient and provided a physical guide that maintained a desired positionand orientation of the tool during its operation.

For example, some prosthetic implants used in knee joint replacementsurgeries comprise projections, keels, and/or other mechanical elementsthat are configured to fit within corresponding holes or voids createdin the bone to secure the implant to the bone. In order to ensure theaccurate placement of these voids, a jig was often used to mechanicallyalign a drill in a desired position and orientation with respect to thebone of a patient. During operation of the drill, the jig would maintainthe desired orientation while the surgeon advanced the drill into to thebone until the desired depth was reached.

Although these guide jigs enhanced the accuracy and precision of theplacement of voids within the bone, they needed to be physicallyinstalled in proximity to the bone during the surgical procedure. Theaccurate alignment and placement of these guides can take a considerableamount of time, which could prolong the surgical procedure. Furthermore,mechanical jigs and cutting guides are typically too large to fit withinthe relatively small spaces allowed for minimally-invasive procedures.

With the advent of computer-assisted surgery (CAS) systems, surgeonswere no longer required to rely on mechanical jigs for precisionpositioning of surgical instruments. Specifically, many CAS systemsinclude surgical navigation and tracking software that displays agraphical representation of the surgical site. Using the navigation andtracking features of the CAS system, the surgeon can view the locationof a surgical instrument relative to the patient's anatomy. Using thegraphical interface as a guide, the surgeon can manually navigate thesurgical tool to a desired position within the surgical site.

More sophisticated CAS systems are configured for interactive couplingwith the surgical tools. These CAS systems may be equipped with forcefeedback controls that provide the surgeon with haptic feedback when,for example, the surgical tool interacts with certain pre-establishedvirtual boundaries. Such virtual boundaries may be established toconstrain the surgical instrument from undesired interactions withcertain areas of the patient's anatomy. By strategically arranging thevirtual boundaries for the force feedback controls, users can create“virtual” guides that define the areas in which the tool can operate, aswell as areas that prohibit tool operation. If a surgical procedurerequires the drilling of a post hole in a patient's bone, a virtualboundary may be established to define the desired position, orientation,and size of the hole. The virtual boundary may constrain a surgical toolfrom operating outside of the established boundary.

Although existing virtual guide methods provide a solution for definingthe areas of allowed operation (and corresponding areas of constrainedoperation) of a surgical instrument, they may still be inefficient. Forexample, conventional virtual guide methods do include a solution foraligning a surgical tool in a proper orientation prior to engagementwith the patient's anatomy. As a result, in surgical procedures thatrequire precision cuts having specific orientations (such as thedrilling of post or guide holes within bone), the surgeon may berequired to manually “search” for the appropriate orientation by usingthe tip of the surgical tool as an exploring device to first locate theengagement point at the surface of the patient's bone. Once theengagement point has been located, the surgeon then manually pivots thesurgical tool to locate the appropriate orientation for advancing thetool to the target point. Not only is such a manual process frustratingto the surgeon, it may unnecessarily prolong the surgery, which canincrease costs.

The presently disclosed systems and methods for guiding a surgicalinstrument to a target orientation and/or position are directed toovercoming one or more of the problems set forth above and/or otherproblems in the art.

SUMMARY

According to one aspect, the present disclosure is directed to acomputer-implemented method for guiding an instrument. The method maycomprise determining, by a processor associated with a computer, anorientation angle of an instrument axis relative to a target axis. Themethod may also comprise establishing, by the processor, a haptic forceassociated with the instrument based on the determined orientation angleof the instrument axis relative to the target axis. The haptic force maybe configured to constrain the instrument axis from being moved to anangle substantially greater than the orientation angle.

In accordance with another aspect, the present disclosure is directed toa method for guiding an instrument, comprising establishing, by aprocessor associated with a computer, a target axis. The method may alsocomprise determining, by the processor, a current orientation angle ofthe instrument relative to the target axis. The current orientationangle may be compared with a previous orientation angle relative to thetarget axis. If the current orientation angle is less than the previousorientation angle, a virtual haptic surface may be established relativeto the target axis. The established virtual haptic surface may comprisea surface angle less than the previous orientation angle.

According to another aspect, the present disclosure is directed to analternative method for guiding an instrument, comprising establishing atarget axis that comprises a target point. The method may also comprisedefining a virtual haptic volume based, at least in part, on the targetpoint and the target axis. The method may further comprise determining aposition of a reference point of the instrument. Upon determining thatthe position of the instrument reference point is within the virtualhaptic volume, the virtual haptic volume may be decreased based on adetection of a corresponding decrease in an orientation angle of theinstrument axis relative to the target axis.

In accordance with yet another aspect, the present disclosure isdirected to a computer-assisted surgery system comprising a surgicaldevice for performing at least one task associated with a surgicalprocedure and a processor, operatively coupled to the surgical device.The processor may be configured to determine an orientation angle of aninstrument axis relative to a target axis. The processor may also beconfigured to apply a haptic force to the surgical device based on thedetermined orientation angle of the instrument axis relative to thetarget axis. The haptic force may be configured to constrain theinstrument axis from being moved to an angle substantially greater thanthe orientation angle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a perspective view of an exemplary surgical procedurethat may be performed consistent with the disclosed embodiments;

FIG. 2 provides a schematic illustration of an exemplarycomputer-assisted surgery (CAS) system, in which certain methodsconsistent with the disclosed embodiments may be implemented;

FIG. 3 provides a schematic diagram of an exemplary computer system,which may be used in one or more components associated with the CASsystem illustrated in FIG. 2;

FIG. 4 provides an illustration of an exemplary virtual haptic volume,consistent with certain disclosed embodiments;

FIG. 5 provides a 2-dimensional side view of a virtual haptic volume,consistent with the disclosed embodiments;

FIG. 5A provides a side view of an exemplary revolute shape that, whenrotated about a target axis, defines the virtual haptic volumeillustrated in FIG. 5;

FIG. 6 provides a flowchart showing an exemplary method for establishinga virtual haptic surface, in accordance with certain disclosedembodiments;

FIG. 7 provides a flowchart depicting an exemplary method for guiding aninstrument using a virtual haptic geometry, consistent with certaindisclosed embodiments; and

FIG. 8 illustrates an exemplary screen shot of a user interface that maybe displayed during a surgical procedure, in accordance with certainexemplary disclosed embodiments.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of thepresent disclosure, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or similarparts.

FIG. 1 illustrates an exemplary surgical environment 100 in whichprocesses and methods consistent with the disclosed embodiments may beemployed. As illustrated in FIG. 1, many surgical procedures, such asknee replacement procedures, require accurate and precise modificationto the patient's anatomy. One such example is the placement of post orguide holes 102 within a patient's femur 101 using a surgicalinstrument, such as drill (not shown), having a rotary cutting tool 103.Because these post or guide holes 102 correspond to projections on aprefabricated prosthetic implant (not shown), each hole should generallybe accurately and precisely placed at a specific location, depth, andorientation within the patient's bone.

In order to ensure efficient and proper alignment of the post holeswithin the patient's anatomy, a computer-aided surgery (CAS) system maybe used to generate a graphical representation of the surgical site anda corresponding virtual guide that may aid the surgeon in properlyaligning the tool prior to interaction with patient's anatomy. The CASsystem consistent with the present disclosure may also provide a hapticfeedback geometry that captures the surgical tool while the toolapproaches the engagement site. Once captured, the boundaries of thevirtual haptic geometry may limit or restrict the movement of thesurgical instrument within the confines of a haptic volume defined bythe virtual haptic geometry. Based on the surgeon's movements, thehaptic volume may be gradually reduced, limiting the range of motion ofthe surgical instrument until the surgical tool is aligned with thetarget access associated with the post holes. Systems and methods foraligning the surgical instrument consistent with the disclosedembodiments are discussed in greater detail below and in theaccompanying drawings.

As explained, many CAS systems include software that allows users toelectronically register certain anatomic features (e.g., bones, softtissues, etc.), surgical instruments, and other landmarks associatedwith the surgical site. CAS systems may generate a graphicalrepresentation of the surgical site based on the registration of theanatomic features. The CAS software also allows users to plan certainaspects of the surgical procedure, and register these aspects fordisplay with the graphical representation of the surgical site. Forexample, in a knee joint replacement procedure, a surgeon may registertarget navigation points, the location and depth of bone and tissuecuts, virtual boundaries that may be associated with a correspondingreference for the application of haptic force, and other aspects of thesurgery.

FIG. 2 provides a schematic diagram of an exemplary computer-assistedsurgery (CAS) system 200, in which processes and features associatedwith certain disclosed embodiments may be implemented. CAS system 200may be configured to perform a wide variety of orthopedic surgicalprocedures such as, for example, partial or total joint replacementsurgeries. As illustrated in FIG. 2, CAS system 200 may comprise atracking system 201, computer-assisted navigation system 202, one ormore display devices 203 a, 203 b, and a robotic arm 204. It should beappreciated that CAS system 200, as well as the methods and processesdescribed herein, may be applicable to many different types of jointreplacement procedures. Although certain disclosed embodiments may bedescribed with respect to knee replacement procedures, the concepts andmethods described herein may be applicable to other types of orthopedicsurgeries, such as partial hip replacement, full or partial hipresurfacing, shoulder replacement or resurfacing procedures, and othertypes of orthopedic procedures.

Robotic arm 204 can be used in an interactive manner by a surgeon toperform a surgical procedure, such as a knee replacement procedure, on apatient. As shown in FIG. 2, robotic arm 204 includes a base 205, anarticulated arm 206, a force system (not shown), and a controller (notshown). A surgical tool 210 (e.g., an end effector having an operatingmember, such as a saw, reamer, or burr) may be coupled to thearticulated arm 206. The surgeon can manipulate the surgical tool 210 bygrasping and manually moving the articulated arm 206 and/or the surgicaltool 210.

The force system and controller are configured to provide control orguidance to the surgeon during manipulation of the surgical tool. Theforce system is configured to provide at least some force to thesurgical tool via the articulated arm 206, and the controller isprogrammed to generate control signals for controlling the force system.In one embodiment, the force system includes actuators and abackdriveable transmission that provide haptic (or force) feedback toconstrain or inhibit the surgeon from manually moving the surgical toolbeyond predefined virtual boundaries defined by haptic objects asdescribed, for example, in U.S. Pat. No. 8,010,180 and/or U.S. patentapplication Ser. No. 12/654,519 (U.S. Patent Application Pub. No.2010/0170362), filed Dec. 22, 2009, each of which is hereby incorporatedby reference herein in its entirety. According to one embodiment, CASsystem 200 is the RIO® Robotic Arm Interactive Orthopedic Systemmanufactured by MAKO Surgical Corp. of Fort Lauderdale, Fla. The forcesystem and controller may be housed within the robotic arm 204.

Tracking system 201 may include any suitable device or system configuredto track the relative locations, positions, orientations, and/or posesof the surgical tool 210 (coupled to robotic arm 204) and/or positionsof registered portions of a patient's anatomy, such as bones. Suchdevices may employ optical, mechanical, or electromagnetic pose trackingtechnologies. According to one embodiment, tracking system 201 maycomprise a vision-based pose tracking technology, wherein an opticaldetector, such as a camera or infrared sensor, is configured todetermine the position of one or more optical transponders (not shown).Based on the position of the optical transponders, tracking system 201may capture the pose (i.e., the position and orientation) information ofa portion of the patient's anatomy that is registered to thattransponder or set of transponders.

Navigation system 202 may be communicatively coupled to tracking system201 and may be configured to receive tracking data from tracking system201. Based on the received tracking data, navigation system 202 maydetermine the position and orientation associated with one or moreregistered features of the surgical environment, such as surgical tool210 or portions of the patient's anatomy. Navigation system 202 may alsoinclude surgical planning and surgical assistance software that may beused by a surgeon or surgical support staff during the surgicalprocedure. For example, during a joint replacement procedure, navigationsystem 202 may display images related to the surgical procedure on oneor both of the display devices 203 a, 203 b.

Navigation system 202 (and/or one or more constituent components of CASsystem 200) may include or embody a processor-based system (such as ageneral or special-purpose computer) in which processes and methodsconsistent with the disclosed embodiments may be implemented. Forexample, as illustrated in FIG. 3, CAS system 200 may include one ormore hardware and/or software components configured to execute softwareprograms, such as, tracking software, surgical navigation software, 3-Dbone modeling or imaging software, and/or software for establishing andmodifying virtual haptic boundaries for use with a force system toprovide haptic feedback to surgical tool 210. For example, CAS system200 may include one or more hardware components such as, for example, acentral processing unit (CPU) (processor 231); computer-readable media,such as a random access memory (RAM) module 232, a read-only memory(ROM) module 233, and a storage device 234; a database 235; one or moreinput/output (I/O) devices 236; and a network interface 237. Thecomputer system associated with CAS system 200 may include additional,fewer, and/or different components than those listed above. It isunderstood that the components listed above are exemplary only and notintended to be limiting.

Processor 231 may include one or more microprocessors, each configuredto execute instructions and process data to perform one or morefunctions associated with CAS system 200. As illustrated in FIG. 2,processor 231 may be communicatively coupled to RAM 232, ROM 233,storage device 234, database 235, I/O devices 236, and network interface237. Processor 231 may be configured to execute sequences of computerprogram instructions to perform various processes, which will bedescribed in detail below. The computer program instructions may beloaded into RAM for execution by processor 231.

Computer-readable media, such as RAM 232, ROM 233, and storage device234, may be configured to store computer-readable instructions that,when executed by processor 231, may cause CAS system 200 or one or moreconstituent components, such as navigation system 202, to performfunctions or tasks associated with CAS system 200. For example, computerreadable media may include instructions for causing the CAS system 200to perform one or more methods for determining changes in parameters ofa hip joint after a hip arthroplasty procedure. Computer-readable mediamay also contain instructions that cause tracking system 201 to capturepositions of a plurality of anatomical landmarks associated with certainregistered objects, such as surgical tool 210 or portions of a patient'sanatomy, and cause navigation system 202 to generate virtualrepresentations of the registered objects for display on I/O devices236. Exemplary methods for which computer-readable media may containinstructions will be described in greater detail below. It iscontemplated that each portion of a method described herein may havecorresponding instructions stored in computer-readable media for causingone or more components of CAS system 200 to perform the methoddescribed.

I/O devices 236 may include one or more components configured tocommunicate information with a user associated with CAS system 200. Forexample, I/O devices 236 may include a console with an integratedkeyboard and mouse to allow a user (e.g., a surgeon) to input parameters(e.g., surgeon commands 250) associated with CAS system 200. I/O devices236 may also include a display, such as monitors 203 a, 203 b, includinga graphical user interface (GUI) for outputting information on amonitor. I/O devices 236 may also include peripheral devices such as,for example, a printer for printing information associated with CASsystem 236, a user-accessible disk drive (e.g., a USB port, a floppy,CD-ROM, or DVD-ROM drive, etc.) to allow a user to input data stored ona portable media device, a microphone, a speaker system, or any othersuitable type of interface device. For example, I/O devices 236 mayinclude an electronic interface that allows a user to input patientcomputed tomography (CT) data 260 into CAS system 200. This CT data maythen be used to generate and manipulate virtual representations ofportions of the patient's anatomy (e.g., bones) in software.

Processor 231 associated with CAS system 200 may be configured toestablish a virtual haptic geometry associated with or relative to oneor more features of a patient's anatomy. As explained, CAS system 200may be configured to create a virtual representation of a surgical sitethat includes, for example, virtual representations of a patient'sanatomy, a surgical instrument to be used during a surgical procedure, aprobe tool for registering other objects within the surgical site, andany other such object associated with a surgical site.

In addition to physical objects, CAS system 200 may be configured togenerate virtual objects—objects that exist in a software space andwhich may be useful during the performance of a surgical procedure. Forexample, CAS system 200 may be configured to generate virtual boundariesthat correspond to a surgeon's plan for preparing a bone, such asboundaries defining areas of the bone that the surgeon plans to cut,remove, or otherwise alter. Alternatively or additionally, CAS system200 may define virtual objects that correspond to a desired path orcourse over which a portion of surgical tool 210 should navigate toperform a particular task.

According to one embodiment, CAS system 200 may be configured togenerate a virtual haptic geometry that defines a point, line, surface,or volume in a virtual coordinate space. The virtual haptic geometry maybe associated with a haptic feedback or force system of CAS system 200such that, when a tracked position of a portion of the surgical tool(e.g., an established center point 103 a or tool axis 403 of cuttingtool 103) interacts with the virtual haptic geometry, a haptic forcefeedback is generated and applied to surgical tool 210. FIGS. 4 and 5provide alternate views of an exemplary virtual haptic geometry 400 thatmay be generated consistent with the presently disclosed embodiments.

According to one exemplary embodiment, and as illustrated in FIG. 4,virtual haptic geometry 400 may be a substantially funnel-shaped volumethat is positioned and oriented relative to a patient's anatomy, such asfemur 101. As such, virtual haptic geometry 400 may define a virtualpathway to quickly and efficiently guide and position a surgicalinstrument, such as rotary drill or burr, into a proper alignmentrelative to femur 101 prior to engagement with femur 101. According tothe embodiment illustrated in FIG. 4, virtual haptic geometry maycomprise a substantially cone-shaped portion that converges toward asubstantially cylindrically-shaped portion. The cylindrically-shapedportion may extend toward a target point (402 in FIG. 5), which, in theexample illustrated in FIG. 4, corresponds to the depth of post holes102.

FIG. 5 illustrates a side view of the exemplary virtual haptic geometry400 shown in FIG. 4, being accessed by cutting tool 103. As illustratedin FIG. 5, virtual haptic geometry 400 may be defined about a targetaxis 401 that includes a target point 402. Virtual haptic geometry 400may comprise a boundary surface 404, which may be positioned at aninitial boundary angle, θ_(B), relative to the target axis 401.According to the embodiment shown in FIG. 5, boundary surface 404 maydefine a substantially cone-shaped volume having an initial base radius,r. It should be noted, however, that, although the upper portion ofvirtual haptic geometry 400 is illustrated in certain embodiments ashaving a substantially cone-shaped boundary surface, it is contemplatedthat virtual haptic geometry 400 may include or embody any shapesuitable for guiding a cutting tool 103 toward a target point 402. Forexample, as shown in FIG. 8, boundary surface 404 may define asubstantially curved blending section, which is designed to convergetoward the target axis more aggressively than the substantially linearboundary surface shown in FIG. 5.

Target point 402 may be a user-defined point that corresponds to thetarget destination of at least a portion of cutting tool 103. Accordingto one embodiment, target point 402 may define a target depth 410 ofpost or guide hole 102 of femur 101. As such, target point 402corresponds to a desired depth 410 that a reference point 103 a of tool103 (also referred to herein as a tool center point (TCP)) can reachbefore a haptic feedback force is applied to surgical tool 210.

Target axis 401 may include target point 402 and may serve as a centralaxis about which virtual haptic geometry 400 may be defined. Target axis401 may also define a desired axis for approaching target point 402 withcutting tool 103. As such, target axis 401 may define the axis to whichvirtual haptic geometry 400 converges, and may correspond to the desiredor ideal orientation of approach of surgical tool 403 toward targetpoint 402.

FIG. 5A illustrates an side view of an exemplary revolute shape that isused to create the funnel-shaped virtual haptic geometry 400 illustratedin FIG. 5. As illustrated in FIG. 5, virtual haptic geometry 400 may berotationally symmetric about target axis 401. As such, CAS system 200may be configured to generate virtual haptic geometry 400 by creating arevolute shape that defines the profile of the desired boundary surface404 of virtual haptic geometry 400. Once the shape of the revolute isdefined, CAS system 200 may be configured to virtually rotate therevolute shape about target axis 401, defining boundary surface 404 ofvirtual haptic geometry 400.

As illustrated in FIG. 5A, the funnel-shaped haptic volume shown in FIG.5 may be created by first defining the revolute shape that will berotated about a target axis 401. The shape may comprise a firstsubstantially linear segment 510 that is oriented at an angle thatdefines the desired base radius, r, of the funnel portion. Linearsegment 510 may converge toward target axis 401 as it approaches asecond linear segment 530.

Second linear segment 530 may be parallel with target axis 401 and maybe configured to define the desired radius of the cylindrical portion ofvirtual haptic geometry 400. According to one embodiment, the distancebetween target axis 401 and second linear segment 530 may correspond tothe desired radius of post hole 102 (which corresponds to the radius ofa stabilizing element of a prosthetic implant that is to fit within posthole 103). A substantially curved (or circularly blended) segment 520may be included for providing a gradual transition between first andsecond linear segments 510, 530. This gradual transition limits theperception of an abrupt “edge” that would otherwise be associated with ahaptic surface defined by the intersection of two linear segments.

The revolute shape 500 consistent with the disclosed embodiments mayalso comprise a bottom segment 540 that defines the bottom of targetpost hole. In the embodiment illustrated in FIG. 5A, the bottom segmentis substantially curved about target point 402, corresponding to radiusof the burr associated with cutting tool 103. Once the revolute iscomplete, software associated with CAS system 200 may generate thefunnel-shaped virtual haptic boundary 400 by rotating revolute shape 500about target axis 401.

It is contemplated, however, that, as an alternative or in addition tothe funnel-shaped haptic volume shown in FIG. 5 (and the correspondingrevolute shown in FIG. 5A for generating the same) are exemplary onlyand not intended to be limiting. Indeed, CAS system 200 may provideadditional and/or different revolute shapes and processes for creatingcollapsing virtual haptic geometries for guiding surgical instrumentstoward a planned target orientation and target point.

During operation of CAS system 200 and in accordance with an exemplaryembodiment, virtual haptic geometry 400 becomes associated with cuttingtool 103 when reference point 103 a of cutting tool 103 enters thevolume defined by virtual haptic geometry. Once active, virtual hapticgeometry 400 may be configured to provide a haptic feedback when cuttingtool 103 interacts with one or more virtual boundaries 404. For example,virtual haptic geometry 400 may define a haptic “wall” that constrains,inhibits, or prevents cutting tool 103 and/or reference point 103 a frommoving beyond the boundary surface 404 of virtual haptic surface 400. Inan exemplary embodiment, virtual haptic geometry 400 may be configuredwith an “opening” for allowing cutting tool 103 to disengage fromvirtual haptic geometry 400. While this disengagement opening may belocated anywhere along virtual haptic geometry 400, an exemplaryembodiment includes the opening along the surface of virtual hapticgeometry 404 that is located farthest from bone or tissue surfaces ofthe patient. In the embodiment illustrated in FIG. 5, the top surface ofboundary surface 404 may be configured as the disengagement opening.

As an alternative or in addition to constraining the movement of cuttingtool 103 (and/or tool reference point 103 a) from the volume defined byvirtual haptic geometry 400, CAS system 200 may be configured to guidecutting tool 103 toward a desired orientation prior to engagement withfemur 101. Specifically, CAS system 200 may be configured to monitor atool orientation angle, OT, which comprises an orientation angle of thetool axis 403 that is determined based on an orientation of a targetaxis 401. According to one embodiment, such as that illustrated in FIG.5 in which the tool axis 403 is located within the same plane as thetarget axis 401, the orientation angle, θ_(T), which comprises the anglebetween tool axis 403 and target axis 401.

However, when tool axis is not located in the same plane as target axis401 (as it is in FIG. 5), the tool orientation angle, θ_(T), comprisesthe angle formed by the tool axis and a line parallel to target axis 401that is co-planar with tool axis. According to the exemplary embodiment,the line parallel to target axis may be normal to a plane that is normalto target axis 401. In this embodiment, although the tool orientationangle, θ_(T), is not directly determined with respect to the targetaxis, it is based on the relative orientation of target axis withinvirtual software space. As will be explained in further detail below,CAS system 200 may be configured to facilitate alignment of the toolaxis 403 with target axis 401 by modifying the location of boundarysurface 404 of virtual haptic geometry 400 based on the location of toolaxis 403.

To ensure that cutting tool is positioned in the proper orientationprior to engagement with a surface of the patient's anatomy, anintermediate haptic threshold may be established and associated withvirtual haptic boundary 400. Specifically, the virtual haptic boundary400 may include an intermediate tool stop haptic plane 420 that provideshaptic feedback if reference point 103 a attempts to advance withoutbeing in the proper orientation. The haptic feedback may include ahaptic wall that constrains advancement of cutting tool 103 pastintermediate tool stop haptic plane 420 if one or more of tool axis 403and/or tool reference point 103 a is not aligned with target axis 401.

As explained, software associated with CAS system 200 may be configuredto register and track certain aspects of surgical environment 100. Forexample, cutting tool 103, along with other features of surgicalenvironment 100, may be registered and associated with a virtualcoordinate space for tracking and display by CAS system 200. As such,tracking system 201 of CAS system 200 can determine the relativeposition and orientation of cutting tool 103 in the virtual coordinatespace.

In order to properly monitor the orientation of tool axis 403 of cuttingtool 103, the tool axis 403 of cutting tool 103 may first be registeredfor tracking in virtual coordinate space of CAS system 200. According toone embodiment (such as that illustrated in FIG. 5), tool axis 403 maycorrespond to the central axis of a rotary burr, which passes throughreference point 103 a associated with the center of the tip of therotary burr. Tool axis 403, along with reference point 103 a, may bedetermined during a calibration process of CAS system 200, prior to thesurgical procedure. Alternatively or additionally, CAS system 200 may beregistered as part of the registration process during the surgicalprocedure by using a pre-calibrated registration probe to capture andrecord the locations of a plurality of points along tool axis 403.Because the position of the axis at the surface of thecylindrically-shaped rotary burr is slightly different than the positionof the axis at the true center of tool axis 403, CAS system 200 may beprovided with an offset to project the tool axis 403 to the center ofcutting tool 103.

According to yet another embodiment, the pre-calibrated registrationprobe may be used to capture a large number of points along the surfaceof cutting tool 103. Based on the relative locations of these points,software associated with CAS system 200 may be configured to derive toolaxis 403. It is contemplated that additional and/or different methodsmay be used for registering various aspects of cutting tool 103 thanthose that are listed above. For example, a virtual software modelrepresenting cutting tool 103 may be generated using computed tomography(CT) scan information. The model may be registered in the virtualcoordinate space using the calibrated registration probe. Onceregistered, tracking system 201 associated with CAS system 200 canmonitor the real-time location, position, and orientation of registeredcomponents of surgical environment 100 relative to the establishedvirtual coordinate space in order to guide cutting tool 103 to targetdepth points 402 a-402 c by sequentially activating each of target depthpoints 402 a-402 c in accordance with the processes and methodsconsistent with the disclosed embodiments.

Processes and methods consistent with the disclosed embodiments providea solution for quickly and efficiently guiding cutting tool 103 to aproper orientation for engagement with a patient's anatomy. Featuresconsistent with the disclosed embodiments track the position andorientation of a tool axis based on the relative orientation of targetaxis 401. As explained, CAS system 200 may be configured to monitor atool orientation angle, θ_(T), which comprises an orientation angle ofthe tool axis 403 that is determined based on an orientation of a targetaxis 401. According to one embodiment, such as that illustrated in FIG.5 in which the tool axis 403 is located within the same plane as thetarget axis 401, the orientation angle, θ_(T), which comprises the anglebetween tool axis 403 and target axis 401.

However, when tool axis is not located in the same plane as target axis401 (as it is in FIG. 5), the tool orientation angle, θ_(T), comprisesthe angle formed by the tool axis and a line parallel to target axis 401that is co-planar with tool axis. According to the exemplary embodiment,the line parallel to target axis may be normal to a plane that is normalto target axis 401. In this embodiment, although the tool orientationangle, θ_(T), is not directly determined with respect to the targetaxis, it is based on the relative orientation of target axis withinvirtual software space. As will be explained in further detail below,CAS system 200 may be configured to facilitate alignment of the toolaxis 403 with target axis 401 by modifying the location of boundarysurface 404 of virtual haptic geometry 400 based on the location of toolaxis 403.

Exemplary methods consistent with the disclosed embodiments track theposition and orientation of tool axis 403 based on an orientation oftarget axis 401. As the orientation angle, θ_(T), becomes smaller,virtual haptic geometry 400 associated with surgical tool 210 isrepositioned behind the virtual representation of cutting tool 103,creating a boundary surface that “collapses” as cutting tool 103 isbrought into alignment with target axis 401. As a result, cutting tool103 is discouraged, constrained, or prohibited from being rotated to anangle that is substantially greater than the smallest registeredorientation angle. This “collapsing” virtual haptic geometry allows thesurgeon the freedom to move or rotate cutting tool 103 only to thosepositions that bring cutting tool 103 in closer alignment with targetaxis 401. Eventually, as the surgeon rotates cutting toot 103 within thearea defined by virtual haptic geometry 400, the boundary decreases insize until tool axis 403 is sufficiently aligned with target axis 401.FIGS. 6 and 7 provide flowcharts that illustrate exemplary methods forguiding a surgical instrument using haptic feedback consistent with thepresently disclosed embodiments.

It is contemplated that, in accordance with certain embodiments, thecollapsing cone virtual haptic boundary is configured to constraincutting tool 103 from being moved to an angle that is substantiallygreater than the smallest registered orientation angle. Specifically,force feedback system may include or embody an impedance-type forcefeedback system that includes haptic boundaries having variable levelsof perceived “stiffness.” In one embodiment, the stiffness of virtualhaptic boundary may be high, resulting in a haptic boundary thatstrictly constrains cutting tools from being moved to any angle this isgreater than the smallest registered orientation angle.

In certain other embodiments, however, the stiffness of the virtualhaptic boundary may be lowered to provide a “softer” haptic boundary.The resulting haptic boundary may constrain cutting tool from beingmoved to angles that are substantially greater than the smallestregistered orientation. In this embodiment, virtual haptic boundary mayallow some flexibility for cutting tool to exceed the virtual hapticboundary, while substantially constraining cutting tool in accordancewith the virtual haptic boundary.

FIG. 6 provides a flowchart 600 illustrating an exemplary method forguiding a surgical instrument, in accordance with certain disclosedembodiments. As illustrated in FIG. 6, the process comprisesestablishing a target axis corresponding to a desired approachorientation for the tool (Step 610). According to one embodiment, targetaxis 401 may be established during the process of planning the placementof post holes 102 for receiving corresponding stabilizing projections(not shown) associated with a prosthetic implant. During the planningphase, a number of parameters associated with each of post holes 102 maybe established such as, for example, the depth and diameter of the hole,the position of opening of the holes relative to the surface of thebone, and the orientation of hole within the bone.

Upon establishing the parameters for creating each of post holes 102,CAS system 200 may establish target point 402 and target axis 401associated with each of post holes 102. Target point 402 may beconfigured to correspond to the depth that reference point 103 a ofcutting tool 103 should reach in order to complete a respective posthole. Similarly, target axis 401 may be generated as the desired or“ideal” approach orientation in which tool axis 403 of cutting toolshould be maintained in order to create each of post holes 102 accordingto the planned parameters associated therewith.

Once target (i.e., planned) axis 401 has been established, a hapticobject (e.g. virtual haptic boundary 400) may be created around targetaxis 401 (Step 620). According to one embodiment, virtual hapticboundary may be generated by rotating a linear shape corresponding tothe desired profile of virtual haptic boundary 400 (such as the revoluteshape shown in FIG. 5A) about target axis 401. In this way, virtualhaptic boundary 400 is rotationally asymmetric about target axis 401.

Upon establishing and activating the haptic object associated withvirtual haptic boundary 400, reference point 103 a may be monitored withrespect to the haptic volume defined by virtual haptic boundary 400(Step 630). As explained, in order to properly monitor the orientationof tool axis 403 of cutting tool 103, reference point 103 a and toolaxis 403 of cutting tool 103 may first be registered for tracking invirtual coordinate space of CAS system 200. Reference point 103 a may beestablished as the center of the tip of cutting tool 103 and may betracked by tracking system 201 of CAS system 200 to estimate, amongother things, the interaction between tip of cutting tool 103 with thepatient's anatomy.

According to one exemplary embodiment, the position of reference point103 a may be used to activate haptic forces associated with virtualhaptic boundary 400. For example, software associated with CAS system200 may activate haptic forces associated with the boundary surface 404of virtual haptic boundary 400 once reference point 103 a is within thevolume defined by virtual haptic boundary 400. Once the haptic forceshave been activated, CAS system 200 may be configured to apply hapticforces to robotic arm 204 to constrain the movement of cutting tool 103in certain situations.

In addition to monitoring the position of reference point 103 a (e.g.,the tip of cutting tool 103) the process comprises monitoring anorientation of a tool axis 403 of cutting tool 103. As explained,cutting tool 103, along with other features of surgical environment 100,may be registered and associated with a virtual coordinate space fortracking and display by CAS system 200. As such, tracking system 201 ofCAS system 200 can determine the relative position and orientation ofcutting tool 103 in the virtual coordinate space.

Upon determining the relative orientation of cutting tool 103, CASsystem 200 may determine an orientation angle of cutting tool 103relative to target axis 401 (Step 640). According to one embodiment,processor 231 associated with CAS system 200 may calculate theorientation angle (θ_(T) of FIG. 5) by determining the angular offsetbetween the tracked position of tool axis 403 and target axis 401 in thevirtual coordinate space.

Once the orientation angle of tool axis 403 has been determined, theestablished haptic boundary (e.g., virtual haptic geometry 400) may bemodified and/or re-generated based on the orientation angle of tool axis403. As explained, the haptic boundary may embody a virtual surface atwhich a haptic force may be applied to cutting tool 103 in response toan interaction of the virtual representation of cutting tool 103 withvirtual surface 404. The haptic boundary may be positioned in virtual(software) space relative to the virtual representation of cutting tool103 so as to constrain cutting tool 103 from being rotated to an anglegreater than the orientation angle of tool axis 403.

As the orientation angle, θ_(T), decreases, the virtual haptic geometry400 associated with cutting tool 103 collapses behind tool axis 403,thereby causing the virtual haptic geometry 400 to converge on targetaxis 401. As such, when the tool orientation angle, θ_(T), is 0 (toolaxis 403 parallel (or co-linear) to target axis 401, virtual hapticgeometry 400 completely surrounds surgical instrument 403, therebylocking surgical instrument 403 into place. According to one exemplaryembodiment, CAS system 200 may apply, if the tool orientation angle,θ_(T), is less than a threshold angle, a haptic force that constrainscutting tool 103 to a position that brings tool axis 403 in parallelwith target axis 401. According to an exemplary embodiment, thresholdangle may be established as an angle between 0.3 and 3 degrees.

Upon establishing the haptic boundary relative to the tool axis ofcutting tool 103, CAS system 200 may be configured to detect aninteraction of cutting tool 103 with the haptic boundary (Step 650).More specifically, CAS system 200 may be configured to detect aninteraction between reference point 103 a associated with the tip ofcutting tool 103 with a boundary surface 404 of the virtual hapticgeometry. Alternatively or additionally, CAS system 200 may beconfigured to detect an interaction between a portion of tool axis 403with the surface of virtual haptic geometry 400.

After detecting interaction of cutting tool 103 with virtual hapticgeometry 400, a haptic force may be applied to the cutting tool 103(Step 650). For example, should a surgeon try to rotate cutting tool 103to an angle greater than the surface angle of virtual haptic geometry400 (which has been repositioned to correspond to the minimum registeredorientation angle of cutting tool 103), force system of CAS system 200may apply a haptic force to discourage, constrain, or prevent thesurgeon from rotating beyond the surface angle of virtual hapticgeometry 400.

The embodiment illustrated and described with respect to FIG. 6 providesan exemplary embodiment for guiding cutting tool 103 into a properorientation (e.g., parallel with target axis 401) for approaching targetpoint 402. FIG. 7 provides a flowchart 700 illustrating anotherexemplary method for guiding a cutting tool 103 to a target orientationand, ultimately, a target point 402.

As illustrated in FIG. 7, the method may commence upon establishing atarget point 402 and target axis 401 in the virtual coordinate systemassociated with surgical environment 100 (Step 705). For example, a user(e.g., a surgeon, surgical technician, etc.) of CAS system 200 maydefine a target point 402 using a graphical user interface associatedwith CAS system 200. Target point 402 may correspond to a location atwhich at least a portion of cutting tool 103 should reach during asurgical procedure. According to the embodiment illustrated in FIG. 5,target point 402 may be defined within a patient's femur 101 as thedesired depth 410 of a post hole 102 that will receive a stabilizingpost of a prosthetic femoral component.

Once target point 402 has been established, target axis 401 may bedefined. As with target point 402, the user of CAS system 200 may definethe target axis 401 using the graphical interface of CAS system 200.Target axis 401 may be selected as the line that corresponds with adesired approach orientation of cutting tool 103 to target point 402. Inthe case of a post hole, this desired approach orientation correspondswith the desired orientation of the hole within the patient's anatomy,which, in turn, may correspond to the orientation of the stabilizingprojections of a prosthetic implant.

Upon establishing the target point 402 and target axis 401, CAS system200 may define a virtual haptic geometry (Step 710). According to oneembodiment, CAS system 200 may generate a funnel-shaped virtual hapticgeometry that is centered around target axis 401 and converges towardtarget point 402. The virtual haptic geometry may comprise asubstantially cone-shaped upper portion that tapers toward asubstantially cylindrically-shaped lower portion, such as illustrated inFIG. 4. Alternatively or additionally, the virtual haptic geometry maycomprise a substantially curved blending section similar to thatillustrated in FIG. 8.

Regardless of its specific shape, virtual haptic geometry 400 comprisesa haptic boundary surface 404 that defines a boundary at which a virtualhaptic force is applied. That is, when cutting tool 103 is located or“captured” within the volume defined by the virtual haptic geometry 400,attempts to move cutting tool 103 outside of the volume will be met byan opposing force applied at the haptic surface by force system of CASsystem 200. Cutting tool 103 may be able to exit the haptic volume at apredetermined exit point that, according to one embodiment, is typicallythe base of the inverted cone shape, directly opposite the direction ofconvergence of virtual haptic geometry 400.

After virtual haptic geometry 400 has been defined, the position of areference point 103 a associated with cutting tool 103 may be determined(Step 715). Specifically, as explained above, tool axis 403 andreference point 103 a may be registered in a virtual coordinate spacerelative to other landmarks associated with surgical environment 100. Assuch, tracking system 201 of CAS system 200 may be configured to monitorthe position of reference point 103 a. This position may be displayed onone or more displays 203 a, 203 b of CAS system 200 for viewing by thesurgeon. Regardless of whether reference point 103 a is viewed ortracked via display(s) 203 a, 203 b, CAS system 200 may electronicallymonitor the position of reference point 103 a in the virtual coordinatespace.

CAS system 200 may compare the position of the tool reference point 103a with a location of a boundary surface 404 of virtual haptic geometryto determine whether tool reference point 103 a is located within thevolume defined by the virtual haptic geometry (Step 720). Specifically,CAS system 200 may compare the relative position of tool reference point103 a within the virtual coordinate system with the relative position ofboundary surface 404. If the tool reference point 103 a is not withinthe volume defined by the virtual haptic geometry (Step 720: No), CASsystem 200 may revert back to step 715 and continue monitoring theposition of tool reference point 103 a.

If, on the other hand, tool reference point 103 a is within the volumedefined by virtual haptic geometry 400 (Step 720: Yes), CAS system 200may determine a current orientation angle (θ_(T)) of cutting tool 103(Step 725). As explained above, tool axis 403 of cutting tool 103 may beregistered in a virtual coordinate space relative to other landmarksassociated with surgical environment 100. As such, tracking system 201of CAS system 200 may be configured to monitor the orientation of toolaxis 403 within the virtual coordinate space. Based on the orientationsof tool axis 403 and orientation of target axis 401 in virtualcoordinate space, CAS system 200 may be configured to determine anorientation angle, θ_(T), of tool axis 403 relative to target axis 401.

CAS system 200 may then be configured to compare the current orientationangle, θ_(T), associated with cutting tool 103 with a previous toolorientation angle (Step 730). According to one embodiment, trackingsystem 201 of CAS system 200 may be configured to monitor and record thecurrent position and orientation of cutting tool 103, while storingpreviously-monitored position and orientation information in memory.Accordingly, CAS system 200 can compare the current position andorientation of cutting tool 103 with a previous position and orientationof cutting tool 103.

If the current tool orientation angle, θ_(T), is not less than theprevious tool orientation (Step 730: No), CAS system 200 may applyhaptic forces to robotic arm 204 to prevent or constrain the movement ofcutting tool 103 from being positioned in an orientation that is furtheraway from the target axis. If, on the other hand, the tool orientationangle, θ_(T), is less than a previous orientation angle (Step 730: Yes),CAS system 200 may modify the haptic geometry to reduce the hapticboundary angle to match the current tool orientation angle, θ_(T) (Step740). More specifically, CAS system 200 may modify the orientationand/or position of boundary surface 404 to correspond with theorientation and/or position of tool axis 403. For example, using theembodiment illustrated in FIG. 5 as an example, CAS system 200 maymodify the position and angle of boundary surface 404 so that boundarysurface 404 collapses behind tool axis 403. In other words, boundarysurface 404 may be repositioned to include tool axis 403. By modifyingthe position of boundary surface 404 relative to a correspondingdecrease in orientation angle of tool axis 403 relative to target axis401, CAS system 200 constrains cutting tool 103 from being rotated at anangle greater than the smallest registered orientation angle of surgicalaxis 403.

CAS system 200 may then determine whether the current tool orientationangle is within a predetermined threshold angle of target axis 401 (Step745). This predetermined threshold may be a user-defined threshold thatrepresents an acceptable amount of deviation between a current tool axis403 and target axis 401. According to one embodiment, the predeterminedthreshold may be established as 0.5°. Thus, CAS system may compare thecurrent orientation angle, θ_(T), with the threshold angle of 0.5°. Ifthe tool axis 403 is not within 0.5° of the target axis 401 (i.e.,θ_(T)<0.5°) (Step 745: No), the process may revert back to step 725.

If, on the other hand, the current tool orientation angle, θ_(T), iswithin the threshold angle (Step 745: Yes), force system of CAS system200 may engage orientation “hold” forces (Step 750). Tool “hold” forcesmay be haptic forces that are designed to constrain or lock cutting tool103 in the current orientation. Accordingly, these hold forces prevent,constrain, or inhibit the tool from being rotated away from the currenttool orientation angle, θ_(T).

Once tool axis 403 has been properly aligned to within a thresholdorientation angle of target axis 401, CAS system 200 may be configuredto release or remove haptic forces associated with intermediate toolstop haptic plane 420 (Step 755). As such, reference point 103 aassociated with cutting tool 103 may be allowed to advance pastintermediate tool stop haptic plane 420 toward the bone engagementsurface, and proceed to the planned target end point 402.

In addition to guiding tool axis 403 of cutting tool 103 to a targetorientation (e.g., parallel with target axis 401), CAS system 200 mayalso be configured to use haptic force control to guide a tool referencepoint 103 a to target axis 401. Specifically, as illustrated in FIG. 7,after CAS system 200 determines that tool reference point 103 is withinthe volume defined by virtual haptic geometry 400 (Step 720: Yes), CASsystem may determine the position of tool reference point 103 a (e.g.,tool control point) (Step 760). As explained, tool reference point 103 amay be registered in the virtual coordinate space of CAS system 200. Assuch, the relative position of reference point 103 a within surgicalenvironment 100 may be tracked by tracking system 201 of CAS system 200.Thus, tracking software associated with CAS system 200 may monitor and,if so configured, display the relative position of reference point 103 awithin the virtual coordinate space.

Once the current position of tool reference point 103 a has beendetermined, CAS system 200 may determine whether reference point 103 ais within the haptic volume defined by virtual haptic boundary 400 (Step765). As explained, tracking system 201 of CAS system 200 may beconfigured to track the registered position of tool reference point 103a within surgical environment 100. Similarly, CAS system 200 may beconfigured to monitor the location, position, and orientation ofboundary surfaces 404 of virtual haptic boundary 300. As such, trackingsoftware associated with CAS system 200 may compare the position ofreference point 103 a of cutting tool 103 with the position of theboundary surfaces 404 associated with virtual haptic boundary. Ifreference point 103 a is not within the haptic volume associated withthe virtual haptic boundary 400 (Step 765: No), the process may revertback to step 760, where CAS system 200 continues to monitor position oftool reference point 103 a.

If, on the other hand, reference point 103 a is within the haptic volumedefined by virtual haptic boundary 400 (Step 765: Yes), CAS system 200may enable haptic forces associated with virtual haptic boundary 400(Step 770). As explained in connection with the exemplary embodiments,haptic forces are conditionally applied to constrain movement of cuttingtool 103 upon detection of an interaction between cutting tool 103 andboundary surface 404 of virtual haptic geometry 400, in order to preventundesired movement or positioning of cutting tool 103 outside of virtualhaptic geometry 400.

Once the haptic forces associated with the haptic volume have beenactivated, CAS system 200 may determine whether tool reference point 103a is attempting to “break” through the boundary surface(s) 404 ofvirtual haptic geometry 400 (Step 775). According to one embodiment,this involves tracking the position of tool reference point 103 a withinthe volume defined by virtual haptic geometry 400. The tracked positionof reference point 103 a may be compared with the position of boundarysurface 404. Tracking system 201 of CAS system 200 may be configured todetermine when the position of reference point 103 a interferes,intersects, or otherwise overlaps with boundary surface 404. As theposition of reference point 103 a interferes with the position ofboundary surface 404, CAS system 200 may be configured to detect thatreference point 103 a is trying to “break” through the haptic boundarydefined by virtual haptic geometry 400.

In situations in which tool reference point 103 a is not trying to breakthrough boundary surface 404 corresponding to virtual haptic geometry200 (Step 775: No), the process may revert back to step 760, where CASsystem 200 continues to monitor position of tool reference point 103 a.If, however, tracking system 201 associated with CAS system 200determines that reference point 103 a is trying to “break” throughboundary surface 404 associated with virtual haptic geometry 400 (Step775: Yes), force system of CAS system 200 may be configured to applycorresponding haptic forces to constrain the tool center point withinthe haptic volume (Step 780). According to one embodiment, CAS system200 is configured as an impedance-type haptic system, whereby hapticforces are designed to simulate a mechanical impedance based on theposition of reference point 103 a relative to boundary surface 404 ofvirtual haptic geometry 400. It is contemplated, however, that, althoughthe exemplary embodiments are described with respect to CAS system 200being embodied as a impedance-type haptic system, that certain processesand methods consistent with the disclosed embodiments are compatiblewith admittance-type haptic force control systems.

It is also contemplated that virtual haptic geometry 400 may beconfigured to constrain movement of the position and orientation ofcutting tool 103 while reference point 103 a is located within thevolume defined by virtual haptic geometry 400. It is contemplated,however, that the virtual haptic geometry 400 (and/or haptic forcesassociated therewith) may be deactivated, for example, by removingcutting tool 103 from the haptic volume associated with virtual hapticgeometry 400 in a predetermined manner, such as by extracting the tip ofcutting tool 103 from the top of virtual haptic geometry 400.

FIG. 8 illustrates an exemplary screen shot 800 associated with agraphical user interface of CAS system 200. As illustrated in FIG. 8,CAS system 200 may provide a graphical user interface that displayscertain aspects associated with surgical environment 100 on one or moredisplays 203 a, 203 b. According to an exemplary embodiment, thegraphical user interface may provide a display of the virtual hapticgeometry 400 relative to one or more features associated with thesurgical environment 100, such as femur 101. As such, CAS system 200 maybe adapted to provide a graphical display that, in addition to thehaptic guidance, can assist the surgeon in guiding cutting tool 103toward a target approach orientation (target axis 401) and, ultimately,a target point 402. During use, the graphical user interface may alsotrack tool axis 403 and tool reference point 103 a of cutting tool 103relative to the other displayed aspects of surgical environment 100.Furthermore, graphical user interface of CAS system 200 may also displayany modifications of virtual haptic geometry 400 that result from thedisclosed methods for guiding cutting tool 103.

It should be noted that, although virtual haptic geometry is illustratedand described in the exemplary embodiments as being a substantiallyfunnel-shaped boundary for use in guiding a surgical drill or burr to atarget point, the presently disclosed embodiments are applicable forgenerating virtual haptic boundaries having different revolute shapesfor use with other types of tools. For example, the presently disclosedembodiments contemplate generating a collapsible virtual haptic geometryhaving a substantially “Y”-shaped cross section having a substantially“V”-shaped upper section that converges to a substantially planar lowersection for guiding a planar or sagittal saw toward a desiredorientation for executing a planar cut on a bone. Consequently, it iscontemplated that the methods described herein may be employed invirtually any environment where it may be advantageous to guide asurgical tool to a predetermined orientation for approaching a targettool operation site.

The presently disclosed systems and methods provide a solution thatenables a computer-assisted surgical system to quickly, efficiently, andaccurately guide a surgical tool to a target approach orientation and,ultimately, to a target point. More specifically, the systems andmethods described herein implement a collapsible virtual haptic geometrythat updates the location of haptic forces in response to real-timemodifications to the position cutting tool 103 to discourage orconstrain the surgeon from moving the cutting tool 103 to a positionand/or orientation that does not converge toward a desired orientation.

CAS systems 200 configured in accordance with the presently disclosedembodiments may have several advantages. For example, by providing acollapsible virtual haptic geometry that only allows orientation of thesurgical tool to be modified in a manner that converges on a targetaxis, the presently disclosed CAS systems 200 may significantly decreasethe amount of time required to properly orient cutting tool 103 to reachits desired target point. This may ultimately decrease the amount oftime required to perform the required surgical tasks, particularly whencompared with conventional techniques that require the surgeon to “find”the proper orientation through trial-and-error searching.

The foregoing descriptions have been presented for purposes ofillustration and description. They are not exhaustive and do not limitthe disclosed embodiments to the precise form disclosed. Modificationsand variations are possible in light of the above teachings or may beacquired from practicing the disclosed embodiments. For example, thedescribed implementation includes software, but the disclosedembodiments may be implemented as a combination of hardware and softwareor in firmware. Examples of hardware include computing or processingsystems, including personal computers, servers, laptops, mainframes,micro-processors, and the like. Additionally, although disclosed aspectsare described as being stored in a memory, one skilled in the art willappreciate that these aspects can also be stored on other types ofcomputer-readable storage devices, such as secondary storage devices,like hard disks, floppy disks, a CD-ROM, USB media, DVD, or other formsof RAM or ROM.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed systems andassociated methods for guiding a surgical instrument to a targetorientation and/or position. Other embodiments of the present disclosurewill be apparent to those skilled in the art from consideration of thespecification and practice of the present disclosure. It is intendedthat the specification and examples be considered as exemplary only,with a true scope of the present disclosure being indicated by thefollowing claims and their equivalents.

1-35. (canceled)
 36. A method for guiding an instrument, comprising:establishing, by a processor associated with a computer, a target axisthat comprises a target point; defining, by the processor, a virtualhaptic volume based, at least in part, on the target point and thetarget axis, the virtual haptic volume comprising an intermediate hapticplane; determining, by the processor, a position of a reference point ofan instrument and an orientation angle of an axis of the instrumentrelative to a plane normal to the target axis; determining, by theprocessor, that at least one of the position of the reference point andthe axis of the instrument is not aligned with the target axis; whereinupon determining that at least one of the position of the referencepoint and the axis of the instrument is not aligned with the targetaxis, the intermediate haptic plane prevents the reference point of theinstrument from moving towards the target point until the referencepoint and the axis of the instrument is aligned with the target axis.37. The method of claim 36, wherein the haptic volume is y-shaped,having a v-shaped upper section that converges to a planar lowersection.
 38. The method of claim 36, wherein the haptic volume isfunnel-shaped, having a cone-shaped upper section that converges to acylindrical lower section.
 39. The method of claim 36, wherein at leasta portion of the haptic volume is cone-shaped.
 40. The method of claim39, wherein the cone-shaped portion intersects a cylindrically-shapedportion, wherein the intersection between the cone-shaped portion andthe cylindrically-shaped portion is blended to form a substantiallycurved surface at the intersection.
 41. The method of claim 36, furthercomprising: determining, by the processor, whether a position of thereference point is within the virtual haptic volume; wherein theorientation angle of the axis of the instrument is determined upondetermining that the position of the reference point is within thevirtual haptic volume; determining, by the processor, a decrease in theorientation angle of the axis of the instrument relative to the planenormal to the target axis; and decreasing, by the processor, the virtualhaptic volume based on the decrease in the orientation angle of the axisof the instrument relative to the plane normal to the target axis. 42.The method of claim 41, wherein decreasing the virtual haptic volumeincludes decreasing a distance of the boundary of the virtual hapticvolume from the target axis.
 43. The method of claim 36, wherein thevirtual haptic boundary is configured to constrain the axis of theinstrument from being moved to an angle substantially greater than thedetermined orientation angle.
 44. The method of claim 36, furthercomprising determining a distance between the reference point of theinstrument and the target axis, the virtual haptic boundary providing ahaptic force configured to constrain the reference point of theinstrument from being positioned at a distance greater than the distancebetween the reference point of the instrument and the target axis. 45.The method of claim 44, further comprising applying, if a distancebetween the reference point of the instrument and the target axis isless than a threshold distance, a second haptic force configured to urgethe reference point to a position along the target axis.